Study on imbibition displacement mechanism of clear fracturing fluid in tight oil reservoir based on 2D NMR

In the process of fracturing in tight reservoirs, the imbibition and displacement of crude oil in reservoir pores by fracturing fluids has gradually become a key research field of enhanced oil recovery technology. However, the production characteristics and mechanism of pore crude oil at different scales in the process of imbibition are still unclear, which seriously restricts the optimal design of fracturing fluid system and the reasonable selection of mining technology. Taking the Chang 7 member tight reservoir in the Ordos Basin as the research object, the amphoteric surfactant (EAB-40) was used as the main agent of the clean fracturing fluid system, combined with T₁-T₂ two-dimensional nuclear magnetic resonance and wettability test, the influence of surfactant concentration on reservoir interface properties and fracturing fluid imbibition and displacement efficiency was systematically studied, and its microscopic mechanism was revealed. The experimental results show that EAB-40 significantly enhances the capillary driving force and crude oil desorption efficiency by synergistically reducing the oil-water interfacial tension (up to the order of 10^-2 mN/m) and inducing the wettability reversal (the contact angle is reduced from 147° to 57.34°). The comprehensive oil displacement effect of the fracturing fluid system is optimal when the concentration of surfactant is 0.1 wt%. During the imbibibibition process, the wettability inversion is caused by the concentration of water-wet minerals in the small pores, and the diffusion of surfactants causes the wetting inversion, which drives the crude oil to migrate efficiently from the small pores T₂＜1 ms to the middle (T₂ is between 1 and 100 ms) and large pores T₂＞100 ms. Polymer molecules improve the rheological properties of the fracturing fluid system and promote the deep utilization of residual oil in bound oil and blind end pores. Realize the triple synergistic imbibibibibition mechanism of “IFT reduction-wetting inversion-viscoelastic flow control”.